What is the relation between Time Remapping, FPS, and the number of frames?
A ratio of 100/125 means you want to stretch your action by a quarter, it keeps the exact same action but plays it 1/4 slower (and not faster!).
Note that the Time Remapping values dont affect the duration of the rendered movie, the Timeline still does it.
Let say your action finishes at the end of your Timeline. In the case of a slowed action, if you want to see the full action, and don't want it to be cut off by a quarter, you'll also need to increase the Timeline length by a quarter.
If you want to speed up your action by a quarter you need to set a ratio of 100/75, in that case your action will be finished at 75% of the rendered movie, you can either let it as it is (the last quarter will be still), or cut off 1/4 of your timeline, or cut off the last 1/4 of the rendered movie.
In both case you don't have to change the fps, Time Remapping precisely allows you to ignore the fps!